The strength and weight of an aero gas turbine engine low pressure turbine (LPT) system is governed by the maximum turbine speed (terminal speed) that can occur after failure of the low pressure (LP) shaft connecting the LPT and the LP torque-receiving unit (typically an LP compressor or a fan). Functionality can be built into the engine control system to detect shaft breakage and take accommodation actions (e.g. shut-off fuel flow to the engine) to reduce the turbine terminal speed.
For example, rotational frequency sensors can be provided at each end of the shaft, e.g. as described in U.S. Pat. No. 6,494,046. There may be eight such sensors in total, with four measuring compressor speed and four measuring turbine speed for a dual-channel redundant control system. The processed signals can determine a measure of twist within the shaft. When the twist exceeds a threshold (for example 25°), accommodation actions are triggered.
FIG. 1 shows plots for the same engine spool of: modelled physical (actual) shaft twist against time, and shaft twist against time as determined by a turbine overspeed (TOS) shaft break detection system which conservatively over-estimates the amount of twist. To deliver an 18 ms detection time (corresponding to 25° on the TOS twist plot), the TOS detection system needs to trip when the actual twist in the shaft is only around 7°. The plateau at 30° on the TOS twist plot is simply the upper limit of the detection system.
TOS detection systems may be susceptible to “false positive” spurious detections following engine events such as surge and ice ingestion. For example, FIG. 2 shows a plot of shaft twist against time according to a TOS detection system interpretation of the signals from four sets of frequency sensors, the plot encompassing an ice shedding manoeuvre starting at about time −0.25 s. The detected twist peaks at about 11°, which is a substantial proportion of the 25° threshold. FIG. 3 shows plots, for the same engine spool and the same ice shedding manoeuvre, of: demanded LP shaft speed (NL Dem), actual LPT speed (NLT), actual LP compressor speed (NLC), the speed in each case being expressed as a percentage of maximum shaft speed. The ice shedding manoeuvre causes the spool to resonate, thereby superimposing oscillatory patterns on NLT and NLC as the shaft speed increases from a lower level to the higher demanded level. FIG. 4 shows plots, for a simplified synthesis of the same engine spool and the same ice shedding manoeuvre, of: modelled physical (actual) shaft twist against time, and shaft twist against time as determined by the TOS detection system. The actual shaft twist during the event is probably less than 2°.
A turbine engine spool can be simulated as two masses coupled by a relatively flexible shaft. When an excitation is applied to the shaft the shaft speed varies approximately sinusoidally at a resonant frequency. The resonant frequency can be determined by simple mechanics from the inertias of the two masses and the shaft stiffness. For a typical large 3-shaft engine, the resonant frequency of the LP shaft may be around 14 Hz. Events such as that illustrated in FIGS. 2 and 3 allow the resonant frequency to be checked. The position of the fan and/or the turbine at a given time can be determined from such a simulation for any resonance amplitude and frequency.
Thus known spurious trip threats cause first order torsional resonance of the LP system. For half of the resonance period these events have a signature that is difficult to distinguish from that of a breakage event. Due to the requirement for rapid shaft break detection, a resonance of relatively modest amplitude can cause spurious detection of breakage. To eliminate the risk of spurious detection using known approaches would require a confirmation period greater than half of the resonance period.
Accordingly, there is a need to provide fast and reliable detection of shaft breakage via a detection system that does not respond to spurious trip threats. One approach for reducing the likelihood of spurious detections would be to allow a longer period of time in which to detect shaft breaks. However, this would lead to an undesirable increase in engine weight.